Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

AUTHORS

Hildegard Westphal � Rosenstiel Schoolfor Marine and Atmospheric Sciences, Universityof Miami, 4600 Rickenbacker Cswy., Miami,Florida 33149; present address: Institute ofPaleontology, Erlangen University, Loewenichstraße28, 91054 Erlangen, Germany;[email protected]

Hildegard Westphal studied geology in Tubingen,Brisbane, and Kiel, where she received her Ph.D.in 1997. After a postdoctoral position at RosenstielSchool for Marine and Atmospheric Sciences, Uni-versity of Miami, she became an assistant professorat Hannover University. Currently, she is a memberof the Paleontology Department at Erlangen University.Her work focuses on early diagenesis of carbonates;the genesis, diagenesis, and paleoenvironmentalrecord of limestone-marl alternations; and paleo-ecological interpretation of carbonate platforms.

Gregor P. Eberli � Rosenstiel School for Marineand Atmospheric Sciences, University of Miami, 4600Rickenbacker Cswy., Miami, Florida 33149;[email protected]

Gregor Eberli received his Ph.D.s from the SwissInstitute of Technology (ETH), Zurich, Switzerland,in 1985 and the University of Miami in 1991. Withhis colleagues at the Comparative SedimentologyLaboratory, he conducts research in sedimentology,stratigraphy, geochemistry, and petrophysics of mod-ern and ancient carbonates. In several projects, heinvestigated the influence of sea level changes onsedimentary architecture. He was an AAPG Distin-guished Lecturer in 1996–1997 and a Joint Oceano-graphic Institutions/U.S. Science Advisory CommitteeDistinguished Lecturer in 1998 –1999.

Langhorne B. Smith � Rosenstiel School forMarine and Atmospheric Sciences, University ofMiami, 4600 Rickenbacker Cswy., Miami, Florida33149; present address: New York State Museum,Room 3124 CEC, Albany, New York 12230;[email protected]

Langhorne Smith currently heads the Reservoir Char-acterization Group at the New York State Museum. Heholds a B.S. degree from Temple University, a Ph.D.from Virginia Tech, and did postdoctoral work at theUniversity of Miami. He also worked for Chevron as adevelopment geologist for two years. His currentresearch interests are in carbonate reservoir charac-terization and hydrothermal alteration of carbonatereservoirs.

G. Michael Grammer � Department of Geo-sciences, Western Michigan University, Kalamazoo,Michigan 49008-5241; [email protected]

Reservoir characterizationof the Mississippian MadisonFormation, Wind Riverbasin, WyomingHildegard Westphal, Gregor P. Eberli,Langhorne B. Smith, G. Michael Grammer,and J. Kislak

ABSTRACT

Significant heterogeneity in petrophysical properties, including var-

iations in porosity and permeability, are well documented from car-

bonate systems. These variations in physical properties are typically

influenced by original facies heterogeneity, the early diagenetic en-

vironment, and later stage diagenetic overprint. The heterogeneities

in the Mississippian Madison Formation in the Wind River basin

of Wyoming are a complex interplay between these three factors

whereby differences from the facies arrangement are first reduced

by pervasive dolomitization, but late-stage hydrothermal diagenesis

introduces additional heterogeneity.

The dolomitized portions of the Madison Formation form highly

productive gas reservoirs at Madden Deep field with typical initial

production rates in excess of 50 MMCFGD. In the study area, the

Madison Formation is composed of four third-order depositional

sequences that contain several small-scale, higher frequency cycles.

The cycles and sequences display a facies partitioning with mud-

stone to wackestone units in the transgressive portion and skeletal

and oolitic packstone and grainstone in the regressive portions. The

grainstone packages are amalgamated tidally influenced skeletal and

oolitic shoals that cover the entire study area. The basal three se-

quences are completely dolomitized, whereas the fourth sequence

is limestone. The distribution of petrophysical properties in the

system is influenced only in a limited manner by the smaller scale

stratigraphic architecture. Porosity and permeability are controlled

by the sequence-scale stratigraphic units, where uniform facies belts

and pervasive dolomitization result in flow units that are basically

tied to third-order depositional sequences with a thickness of 65–

100 ft (20–30 m).

AAPG Bulletin, v. 88, no. 4 (April 2004), pp. 405–432 405

Copyright #2004. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received April 12, 2000; provisional acceptance January 12, 2001; revised manuscriptreceived June 19, 2003; final acceptance December 2, 2003.

The best reservoir rocks are found in regressive, coarse-grained

dolomites of the lower two sequences. Although the amalgamated

shoal facies is heterogeneous, dolomitization decompartmentalized

these cycles. Fine-grained sediments in the basal transgressive parts

of these sequences, along with caliche and chert layers on top of the

underlying sequences, are responsible for a decrease of porosity to-

ward the sequence boundaries and potential flow separation. Good

reservoir quality is also found in the third sequence, which is com-

posed of dolomitized carbonate mud. However, reservoir-quality

predictions in these dolomudstones are complicated by several phases

of brecciation. The most influential of these brecciations is hydro-

thermal in origin and partly shattered the entire unit. The breccia is

healed by calcite that isolates individual dolomite clasts. As a result,

the permeability decreases in zones of brecciation. The late-stage

calcite cementation related to the hydrothermal activity is the most

important factor to create reservoir heterogeneity in the uniform

third sequence, but it is also influential in the grainstone units of the

first two sequences. In these sequences, the calcifying fluids invade

the dolomite and partly occlude the interparticle porosity and de-

crease permeability to create heterogeneity in a rock in which the

pervasive dolomitization previously reduced much of the influence

of facies heterogeneity.

INTRODUCTION

Carbonate reservoirs pose unique problems for exploration and

production because of their complex variations in lithology and

diagenetic history that result in heterogeneous reservoir properties

(Stoudt and Harris, 1994). Optimizing field development in such an

environment requires a level of reservoir characterization that

adequately defines vertical and lateral variations in reservoir quality.

The sequence-stratigraphic framework is fundamental in assessing

flow units, extrapolating well data, and predicting performance

anomalies (Lucia et al., 1995; Weber et al., 1995). Small-scale de-

positional cycles are regarded as the primary flow unit in many

carbonate reservoirs (e.g., Lucia et al., 1995). Diagenetic reorgani-

zation of pore systems in many cases crosscuts the stratigraphic

boundaries between small-scale cycles, requiring an integrated stra-

tigraphic and diagenetic reservoir model (Lucia, 1983, 1995, 1999).

The reservoirs of the Mississippian Madison Formation at

Madden Deep field in the Wind River basin, Wyoming, exhibit

characteristics of cyclic carbonate deposition with a strong diagen-

etic overprint (Moore, 2001). The Madden Deep field, with pro-

ducing intervals in a depth of approximately 24,000 ft (7300 m), is

the one of the deepest gas fields on the crest of the Madden anti-

cline. The project presented here was initiated by a plan to expand

the field by drilling wells farther downdip on the anticline. Although

three-dimensional seismic data existed, the resolution at the res-

ervoir level was insufficient to evaluate reservoir continuity and

G. Michael Grammer is an associate professor atWestern Michigan University. His research includeshigh-resolution carbonate sequence stratigraphy andearly diagenesis and their application to reservoir char-acterization. He was an AAPG Distinguished Lecturerfor 2002 –2003 and is a coleader of AAPG’s moderncarbonate field course. Previously, he was a seniorresearch associate for Texaco and has consulted fordomestic and international oil companies. He receivedhis Ph.D. from the University of Miami in 1991.

J. Kislak � Rosenstiel School for Marine andAtmospheric Sciences, University of Miami, 4600Rickenbacker Cswy., Miami, Florida 33149

Jason Kislak received his bachelor’s degree fromFranklin & Marshall College in Lancaster, Pennsyl-vania. He is currently working toward his master’sdegree at the Rosenstiel School of Marine andAtmospheric Science at the University of Miami.

ACKNOWLEDGEMENTS

We are indebted to Connie Hawkins (then with LL&E),who initiated and monitored this project. He helpedsecure the funding, introduced us to the study areas,and provided us with logistical support. We thankBurlington Resources, Texaco North American Pro-ducing West (Denver) and Texaco Upstream Tech-nology (Houston), Chevron Research, and TomBrown Inc. for funding the project on the MadisonFormation. We thank Hutch Jobe (BurlingtonResources) for providing us with data on the MaddenDeep field. Our fieldwork would have been impos-sible without the support of the Burlington FieldStation at Lysite. We are indebted to the Wind RiverIndian Reservation Authorities (namely, LaJeunesse)for granting permission for working on their land.We thank Kelly Bergman, Karin Bernet, Xavier Janson(all University of Miami) Jose-Luis Massafero (fromUniversity of Miami, now in Shell Research) andChristian Betzler (Hamburg University) for assistancein the field, and Alan Buck (University of Miami) forpreparation of the samples. Part of the porosity andpermeability analyses were performed by TexacoUpstream Technology.Leslie Melim (Western Illinois University) is acknowl-edged for discussions on the petrography. Precisesampling of the calcite cements and the stableisotope measurements were performed in PeterSwart’s laboratory. He also helped with the interpre-tation of these data. Clyde Moore (Colorado Schoolof Mines) contributed to discussions on the dia-genesis. Comments of AAPG referees Emily Stout,Clyde Moore, and William A. Morgan and AAPGeditors Neil Hurley and Rick Erickson greatly helpedto improve this manuscript.

406 Reservoir Characterization of the Mississippian Madison Formation, Wind River Basin

heterogeneity. Earlier studies had demonstrated that

the outcrops and subsurface strata had a similar depo-

sitional and diagenetic history, resulting in comparable

rock fabrics and porosity development (Crockett,

1994; Moore, 1995, 2001). Thus, nearby outcrops in

combination with shallow subsurface core data and

deep subsurface core and wire-line data were used to

assess the reservoir quality and heterogeneities of the

Madison Formation at Madden Deep field and adjacent

areas. The approach of using outcrop analogs as a proxy

for the producing reservoir is also warranted, because the

hostile conditions at deep burial (as much as 24,000 ft

[7300 m]) and limited well control inhibit the assess-

ment of lateral reservoir heterogeneities from subsur-

face data in the Madden Deep field area. In particular,

this study addressed the following questions, which

were important for an expansion of the deep Madison

reservoir at Madden field:

1. What are the depositional environments of the Mad-

ison Formation and its sequences, and how extensive

are the individual facies belts? Do facies variations

contribute to the reservoir heterogeneity?

2. Which stratigraphic units define the elementary flow

unit, the genetic units (small-scale cycles) or lower

frequency third-order sequences?

3. Does dolomitization follow the sequence-stratigraphic

facies distribution patterns?

4. What controls porosity development in the dolo-

mites? Is porosity distribution predictable?

To answer these questions, we integrated sedimen-

tology, sequence stratigraphy, petrography, and petro-

physics of outcrop strata and available subsurface data.

The goal was to assess the potential of stepped out wells

to produce at a level similar to the producing wells.

STUDY AREA AND METHODOLOGY

The study focuses on the pervasively dolomitized Mad-

ison Formation in and around the Madden Deep field

in the Wind River basin of Wyoming (Figure 1). The

Tertiary Owl Creek thrust fault, with a vertical dis-

placement between 35,000 and 40,000 ft (11,000 and

12,000 m), brought the reservoir strata to the surface

about 10 mi (15 km) to the north of the field in the Owl

Creek Mountain Range (Figure 2). These surface expo-

sures display the Madison Formation adjacent to a ma-

jor gas-producing area. Crockett (1994) showed that

the Madison Formation at Madden Deep field and the

nearby outcrops had identical burial and diagenetic

histories until the Eocene Laramide orogeny, when the

outcrops were thrusted to the surface. The subsequent

diagenesis has been relatively minor, making the sur-

face exposures an excellent analog for the deep sub-

surface (Crockett, 1994; Moore, 2001).

Four outcrop areas were studied: Buffalo Creek,

Lysite Mountain, Wind River Canyon, and Owl Creek

(Figure 1B). Each outcrop area provides information

about production-scale heterogeneity, whereas com-

parison of all four areas provides an exploration-scale

assessment of facies distribution. The outcrop areas are

located along the northern margin of the Wind River

basin on a section slightly oblique to the depositional

paleodip. To the north of the Lysite Mountain outcrop,

directly behind the outcrop face, a stratigraphic test well

was drilled by LL&E (Louisiana Land and Exploration)

in 1990 (LL&E 1A Madison Stratigraphic Federal) from

which the entire Madison Formation was successfully

cored (415 ft [127 m] total depth). In addition to this

shallow core, a short core and well logs from the Mad-

den Deep reservoir (BHP Petroleum 2–3 Bighorn) were

included in this study.

In each of the four outcrop areas, we examined

the stratigraphic architecture and facies and petrophys-

ical properties on a small scale with measured sections

approximately at a 300–600-ft (100–200-m) spacing,

with high-resolution sampling from outcrop surface (with

hammer and chisel), and with facies mapping between

these sections. This approach provides detailed infor-

mation on the scale of lateral and vertical variability of

reservoir facies and allows the assessment of the distri-

bution of porosity and permeability and potential flow

barriers in the sequence-stratigraphic framework. These

data were integrated with the data from the shallow-

core LL&E 1A Madison Stratigraphic Federal and then

compared with the deep-core BHP Petroleum 2–3 Big-

horn and its wire-line logs. In particular, data from a

handheld gamma tool along a complete section at Ly-

site Mountain helped correlate between outcrop and

subsurface data.

Petrographic examinations of composition, diage-

netic alterations, and porosity were undertaken for

outcrop and subsurface samples from both cores with

light microscopy and, for selected samples, with scanning

electron microscopy (SEM). Mineralogy was determined

quantitatively using standard x-ray diffractometry. Po-

rosity of outcrop and subsurface samples was deter-

mined by helium injection or weight-volume relation-

ships. Permeability data are based on nitrogen flow

measurements.

Westphal et al. 407

MADISON FORMATION

The Madison Formation was deposited during the

early Mississippian (late Kinderhookian to early Osag-

ean) on an extensive marine ramp that extended from

New Mexico to western Canada (Sando, 1976; Gut-

schick and Sandberg, 1983) (Figure 3). It spans a time

of about 12 m.y., from 357 to 345 Ma (Sonnenfeld,

1996a). At the time of deposition, the study area was

located approximately 5j north of the paleoequator

(McKerrow and Scotese, 1990). In the study area, this

regional ramp was dominated by shallow-marine en-

vironments, but deeper water conditions prevailed

farther west in the Antler foredeep and north in the

Figure 1. (A) Study area with exposures of the Mississippian Madison Formation at the northern margin of the Wind River basin,Wyoming. (pC = Precambrian, Pz = Paleozoic, Mz = Mesozoic, Cz = Cenozoic, Tev = Tertiary volcanics, Qv = Quaternary volcanics)(B) Locations of the Madden field and the four outcrops studied (OC = Owl Creek; WRC = Wind River Canyon; LM = Lysite Mountain;BC = Buffalo Creek). Double line: location of cross section in Figure 4; dashed line: location of regional cross section in Figure 9.


Williston basin and Central Montana trough (Gutschick

and Sandberg, 1983).

The Madison Formation overlies an angular uncon-

formity, and subcrop strata range in age from Precam-

brian in the east to Devonian in the west (Sandberg and

Klapper, 1967). In the study area, the basal unconfor-

mity separates the Madison Formation from the Cam-

brian Gallatin Formation in updip locations (Buffalo

Creek, Lysite) and from the Ordovician Bighorn dolo-

mite in downdip locations (Wind River Canyon, Owl

Creek). The Madison Formation predominantly con-

sists of dolomite and limestone. Some shaly strata occur

at the base of the Madison Formation (Paine shale), and

sporadic quartzose sediments were imported from the

rising Antler highlands (Figure 3) (Budai et al., 1984).

The Madison Formation is bounded at the top by a re-

gional karstified unconformity that may represent a time

span exceeding 20 m.y. (Sando, 1967, 1988). The Madi-

son Formation is overlain by the Pennsylvanian Amsden

sandstone (Darwin member; Figure 4). The Late Creta-

ceous to Eocene Laramide orogeny brought Precambrian

to Tertiary strata, including the Madison Formation, to

the surface (Keefer, 1965), which resulted in the excel-

lent hanging-wall outcrops of the reservoir strata in close

proximity to the Madden Deep gas field (Figure 2).

A high-resolution sequence-stratigraphic frame-

work for the Madison Formation was established by

Sonnenfeld (1996a, b), whose study integrated previous

work by Sando (1976), Peterson (1984, 1987), Elrick

(1990), Elrick and Read (1991), and Crockett (1994).

He determined a fivefold hierarchy of sequences and

cycles: The Madison Formation comprises a second-

order supersequence that is composed of as much as six

third-order sequences (sequences I, II, etc.; Figure 4).

These third-order depositional sequences can be cor-

related over distances of more than 400 mi (640 km)

Figure 2. Schematic cross section through the study areashowing the relationships between the Madison outcrop andwells with shallow core (Lysite Mountain well; completelycored) and deep core on the Madden anticline (BHP Petroleum2–3 Bighorn; partly cored) (after Crockett, 1994).

Figure 3. Paleogeographicreconstruction of the Madisonshelf during the late Osagean(modified from Gutschick andSandberg, 1983; Crockett, 1994).Cenozoic Wind River basin isshown to indicate the studyarea. Outline of the UnitedStates is given for orientationand scale.

Westphal et al. 409

(Sonnenfeld, 1996a, b) and are stacked into two ‘‘third-

order composite sequences,’’ where the first composite

sequence includes third-order sequences I and II, and

the second composite sequence includes sequences III

to VI (Sonnenfeld, 1996a, b). Internally, the third-order

sequences consist of two orders of higher frequency dep-

ositional cycles.

Sonnenfeld (1996a, b) initially picked the maximum

flooding surface of the second-order supersequence to

be coincident with the maximum flooding surface for

third-order sequence I based on the maximum land-

ward occurrence of dark gray, argillaceous, storm-bedded

facies. Recently, Smith et al. (2003) placed the maximum

flooding surface of the second-order supersequence

in the midramp and updip sections at the maximum

flooding surface for sequence IV. Sequence IV is com-

posed of a thick skeletal and oolitic grainstone that marks

a major increase in accommodation space. Our own in-

terpretation corroborates this pick of the maximum

flooding surface in sequence IV because we observe a

large-scale turnaround from aggradation to prograda-

tion at this position.

In the study area, the lower four third-order se-

quences of the Madison Formation are present and have

a total thickness of as much as 350 ft (110 m). These

four third-order sequences can be correlated from the

outcrops into the subsurface at Madden Deep (Crock-

ett, 1994; Moore et al., 1995). Sequences I and II (that

correspond to the first composite sequence of Sonnen-

feld, 1996a, b) are well exposed, whereas outcrops

of sequences III and IV are commonly covered (Fig-

ures 5, 6). The third-order sequences I, II, and III are

dolomitized and are the productive part of the Madi-

son Formation in the subsurface. Most of overlying se-

quence IV is preserved as limestone and may provide a

seal for the underlying dolomitized reservoirs. Laramide

folding and faulting produced favorable source/trap con-

figurations that have led to the large reserve potential of

the Madison Formation in areas such as Madden Deep

field and elsewhere in the Rocky Mountain province.

Figure 4. Stratigraphy of theMississippian strata in the Big-horn basin (Sando and Bamber,1985; Sonnenfeld, 1996a) andin the study area (Wind Riverbasin) (black triangles indicatetransgression, and gray trianglesindicate regressions).


LITHOFACIES ANDDEPOSITIONAL ENVIRONMENT

The area studied is located on the proximal part of the

ancient Madison ramp in shallow-marine conditions.

Most grainstones were deposited in a tidally influenced

environment. Most mudstones and wackestones were

deposited in shallow lagoonal and tidal-flat settings.

Pervasive and fabric-destructive dolomitization

hinders the recognition of depositional facies in some

thin sections, but sedimentary structures in the out-

crop help in the interpretation of depositional envi-

ronment. Dolomites range from tight (interlocking

subhedral dolomite crystals) to sucrosic (euhedral to

subhedral dolomite crystals) with high intercrystalline

porosity. The following is a description and interpre-

tation of each of the important lithofacies of the Mad-

ison Formation in the study area.

Dolomitized Mudstones

1. Massive mudstones: In all of the studied locations,

massive mudstones occur in the lower three third-

order depositional sequences and are, in most cases,

completely dolomitized and show variable inter-

crystalline porosity (Figure 7A, B). These mud-

stones locally contain nodular and/or layered chert,

especially in the Owl Creek location. The deposi-

tional setting for these mudstones is interpreted as a

low-energy ramp.

2. Laminated mudstones: These completely dolomitized

mudstones, in many cases, are intercalated between

grainstone beds. With their parallel laminae, they

are thought to represent tidal-flat laminites.

Dolomitized Algal and Stromatolitic Facies

Dolomitized stromatolitic and cryptalgal rocks are abun-

dant in third-order sequences I and II and the lowermost

part of sequence III. The morphology of the stromatolites

and cryptalgal layers ranges from crinkly laminated,

parallel layers (undulations of millimeter scale) to hemi-

spherical structures (Figure 7C) that are locally stacked

to form buildups. At the base of sequence I at Lysite

Mountain, stromatolites form massive buildups of as

much as 20 ft (7 m) in thickness. Small pores are remi-

niscent of fenestrae. The microstructure of the algal

laminites is obscured by dolomitization. Chert is com-

monly found in algal mudstones, mainly as nodules.

Vugs that are several centimeters in diameter are abun-

dant and are either empty or infilled with large calcite

cement crystals. Similar vugs elsewhere in the Madison

have been interpreted as dissolved evaporite nodules

(Reid and Dorobek, 1993). The algal buildups are in-

terpreted to have formed during times of increased ac-

commodation space (transgression), which provided the

vertical space for these buildups. These buildups are

found in the transgressive parts of high-frequency cy-

cles and are more abundant in the transgressive parts

of third-order sequences.

Dolomitized Wackestones

Dolomitized wackestones and packstones are gener-

ally characterized by ghosts of grains and grain-moldic

porosity (Figure 7D). As in the algal facies, spherical

vugs of several centimeters in diameter are infilled by

large calcite cement crystals and are thought to rep-

resent dissolved evaporite nodules. The wackestones

commonly are massive and lack sedimentary structures

or internal bedding. They are interpreted to be depos-

ited in a restricted marine environment on the shallow

ramp.

Dolomitized Packstones

The packstones range from massive to cross-bedded.

The massive appearance indicates thorough bioturba-

tion in a well-oxygenated, open-marine environment.

The observed cross-bedding documents higher energy

conditions above storm-wave base.

Dolomitized Oolitic Grainstones

In outcrop, dolomitized oolite grainstones appear as pro-

minent, massive, slightly yellow beds as much as 4.5 ft

(1.5 m) in thickness. Individual grainstone layers can be

laterally continuous over 1000 ft (some 300 m) before

they grade into finer sediments. The most common sed-

imentary structure is trough cross-bedding, but herring-

bone cross-stratification is also observed (Figure 7E).

Consequently, grainstones are interpreted to have been

deposited in settings ranging from upper shoreface to

tidal sandbars and, possibly, beach. Unlike other Mis-

sissippian oolites where primary interparticle porosity

is important (Anadarko basin, Hugoton embayment,

Illinois basin, Williston basin; Keith and Zuppann, 1993),

oolitic grainstones in the lower Madison Formation of

the Wind River basin are mainly preserved as dolomites

with significant moldic porosity. In the Owl Creek loca-

tion and in the upper part of the formation at Lysite

Westphal et al. 411

Mountain, some oolitic grainstones are preserved as

low-porosity, tightly cemented limestones (Figure 7F).

Skeletal and Peloidal Grainstones

Skeletal and peloidal grainstones form prominent beds

that appear massive and slightly yellowish in outcrop.

They are mostly dolomitized, but as in the Owl Creek

location, are locally preserved as limestones (Figure 7H).

Most abundant components are brachiopod shells, echi-

noderm debris, peloids, and coated grains. Allochems

are difficult to determine where dolomitized. Cross-

bedding is common, but massive appearance of grain-

stone beds also occurs and is attributed to bioturbation.

Individual beds are 1.5–6 ft (0.05–2 m) thick. Their

lateral continuity increases upward in the sequences,

from isolated sand bodies on the order of 30 ft (10 m) in

lateral extension in the transgressive intervals toward

laterally extensive beds of several hundreds of feet in

the regressive intervals of the sequences. Intercalated,

centimeter-thick, massive, light-colored mudstone layers

are interpreted as storm deposits of winnowed material

that settled from suspension. This facies is probably

equivalent to Sonnenfeld’s (1996b, p. 46) ‘‘cross-strat-

ified skeletal/ooid lime grainstone’’ that is interpreted

to be deposited in an upper shoreface environment.

In association with these grainstones, mud pebbles

are abundant throughout the succession. They accumu-

late in mud pebble conglomerates as rounded to nearly

angular clasts in a muddy matrix. The composition of the

clasts does not differ from the matrix. These mud peb-

bles are interpreted as rip-up clasts of semilithified

carbonate mud reworked during storm events.

Dolomitized Coarse Skeletal Grainstones to Rudstones

Skeletal rudstones are abundant in the upper part of

sequence II at Buffalo Creek and Lysite Mountain,

where they consist of diverse skeletal grains including

red algae, brachiopods, rugose corals, and some pel-

oids. At Buffalo Creek, sequence II rudstones to grain-

stones are strongly dominated by red algal debris (Fig-

ure 8A). The prevalence of corals and algae indicates a

depositional environment of high energy in the vicinity

of coral thickets and red algae buildups.

Bioclastic Floatstones

Floatstones with coarse bioclasts are typical for sequence

IV where they are preserved as limestone. Brachiopods

are the most abundant bioclasts, but echinoderms also

occur in subordinate amounts. The matrix is mud-

stone with locally coarser grained sediment (Figure 8B).

These calcitic floatstones are commonly brecciated, have

high concentrations of stylolites, and are interpreted as

being deposited as wash-over deposits in a protected

lagoon.

Caliche

Thin, laminated, locally reddish calcareous crusts over-

laying beds with sharp boundaries (mostly sequence

boundaries, but also the top of amalgamated grainstone

beds) are interpreted as caliche horizons. In thin sec-

tion, these caliche crusts are characterized by dense

matrix and accessory quartz grains. Conglomeratic tex-

tures are common (Figure 8C). Alveolar textures show a

complex network of micrite tubules, partially filled by

calcite cements. These microscopic features corrobo-

rate the macroscopic interpretation of such crusts as

caliche crusts (Esteban, 1973; Harrison, 1977).

Breccias

1. Small-scale evaporite-solution breccias: A variety of

breccias are observed in the Madison Formation.

The smallest scale breccias are found above sharp

cycle boundaries (in particular sequence boundaries).

These breccias are generally less than 1 ft (30 cm)

thick and show replacement of dolomite by calcite

(dedolomite). In places, this type of breccia consists

predominantly of calcite matrix surrounding clasts

of dolomitized mudstone with strongly corroded do-

lomite rhombs. These breccias are interpreted as early

solution of primary sedimentary evaporite. Evaporite

solution provided the calcium for dedolomitization of

Figure 5. Facies architecture at Buffalo Creek, the most landward location. (A) Panorama of the outcrop, field of view is about 1 mi(1.5 km). The four third-order sequences are clearly distinguished in this outcrop where the lower two sequences form cliffs, the thirdsequence is recessive, and the fourth one forms the cliff at the top of the outcrop. The measured sections are marked. (B) Faciesinterpretation of sequences I and II and part of sequence III. In both sequences, the lower, transgressive portion consists mostly ofmudstone to wackestones with stromatolites, whereas the upper portion of the sequences is dominated by stacked, high-energygrainstone shoals. The first genetic unit of sequence III also shows this facies partitioning.

Westphal et al. 413

the dolomite interpreted as primary in the immediate

vicinity of these evaporites.

2. Evaporite-solution collapse breccia: Monomict mo-

saic breccias to polymict clast-supported breccias

are composed of dolomitic mud clasts and a variety

of cements, whereas sedimentary matrix is scarce

(Figure 8D). The breccias are centimeters to several

meters thick and are composed of millimeter- to

meter-sized clasts of dolomite and limestone. Clasts

have angular shapes and range in size from coarse

sand size to large boulders. This type of breccia oc-

curs predominantly in sequences III and IV. These

breccias are interpreted as collapse of a lithified

formation on top of solution cavities or by dissolu-

tion of thick and massive beds of evaporites and the

collapse of overlying and intercalated strata into the

voids during emergence in the late Mississippian.

The latter possibility has been favored for these brec-

cias in the literature (e.g., Keefer and Lieu, 1966;

Sando, 1967; Moore et al., 1995; Sonnenfeld, 1996,

Smith et al. 2003). The wide lateral extent of these

breccias in outcrop along stratigraphic horizons sup-

ports this interpretation. Late Mississippian evapo-

rites in correlative stratigraphic positions are reported

from westernmost Wyoming (Wanless et al., 1955;

Sando and Dutro, 1960) and northern Wyoming

(Andrichuk, 1955) and are used by Sonnenfeld

(1996a) and Smith et al. (2003) as correlation ho-

rizons between sections.

3. Karst breccias: Karstification from the top of se-

quence IV led to the formation of karst pipes that

penetrate the Madison Formation down to sequence

I. The chaotic breccias that infill these karst cavities

are characterized by a polymictic composition (mas-

sive mudstones, laminated mudstones, chert) and

by a characteristic yellow to reddish silty matrix (Fig-

ure 8E). Karstification is interpreted to have started

shortly after the deposition of the entire Madison

Formation and prior to the deposition of the Ams-

den sandstone (Sando, 1974). During the long-lived

exposure exceeding 20 m.y. (Sando, 1967, 1988),

several karst events could have affected the Madison

Formation.

4. Tectonic breccia: In the Madison Formation we rec-

ognized abundant fracture and brecciated zones that

are calcite cemented. The breccias can be classified

into four categories based on fracture density, calcite

volume, and clast orientation. (1) Fractured breccia

consists of large unrotated clasts and less than 5%

cement. (2) Mosaic breccia has fitted clasts and as

much as 20% cement. (3) Chaotic breccia has rotated

clasts and as much as 80% cement. (4) Shattered

breccia has a high fracture density and less than 5%

calcite cement (Kislak et al., 2001). In the first three

categories, fracture density is proportional to calcite

volume. High volumes of calcite and low fracture

density (fracture, mosaic, chaotic breccia) occur in

the northern section of the thrust sheet. Low volumes

of calcite and high fracture density (shattered breccia)

occur at the leading edge of the thrust sheet. Distri-

bution of the breccias over the length of the thrust

sheet gradually changes from fracture, mosaic, and

chaotic to shattered, whereas distribution in indi-

vidual outcrops is random. The brecciation is most

abundant in the fine-grained mudstone facies of se-

quence III, but it is present throughout the formation.

Brecciation is accompanied by calcite cementation

that heals the fractures induced by tectonic brecciation.

The calcite cements have d18O values between �15

and �25x Peedee belemnite (PDB), which suggests

that the calcite precipitated from hot fluids (Kislak

et al., 2001). Kislak et al. (2001) proposed a hydro-

thermal origin for the breccias based on their morphol-

ogy, distribution, and geochemical signature. The hy-

drothermal activity is related to thrusting during the

Laramide orogeny, when hot fluids from the deeper

part of the thrust sheet migrate along the thrust plane

and locally invade the formation in an explosive man-

ner that creates breccias and fracture zones. In some

places, the breccias follow beds before cutting into

higher stratigraphic levels.

SEQUENCE ARCHITECTURE

Third-Order Depositional Sequences

The four third-order depositional sequences are separat-

ed by surfaces where facies trends of the high-frequency

Figure 6. Facies architecture at Lysite Mountain. (A) Panorama of the outcrop, displaying the four sequences of the MadisonFormation. Field of view is approximately 0.6 mi (1 km). The first two sequences form cliffs, the third one is recessive, and the fourthone forms the cliff at the top of the outcrop. The measured sections are marked. (B) Facies interpretation of sequences I and II and part ofsequence III are shown. The facies architecture is similar to that at Buffalo Creek (Figure 7), with mudstone and wackestone layers and thethick stromatolite layers at the base of sequence I and the amalgamated grainstone layers toward the top of sequences I and II.

Westphal et al. 415

Figure 7. (A) Sucrosic dolostone with high intercrystalline porosity, partly occupied by bitumen (Wind River Canyon, sequence III).(B) Dolomitized mudstone with fracture partly filled with bitumen (Lysite core, sequence III, 149.02 ft below mud pit [fbmp] [45.42 mbelow mud pit (mbmp)]). (C) Dolomitized stromatolitic facies. Hand for scale (Buffalo Creek, sequence I). (D) Tightly dolomitizedwackestone to packstone with relic peloids (Buffalo Creek, sequence I). (E) Oolitic grainstone bed with herringbone cross-bedding(Lysite Mountain, sequence III). Scale in upper left corner (circled) is 20 cm (8 in.) long; (F) Skeletal ooid grainstone preserved aslimestone (Owl Creek, sequence I). (G) Grainstone that has undergone fabric-destructive dolomitization. Because of their sphericalshape and good sorting, these components are most likely ooids (Owl Creek, sequence II). (H) Skeletal-peloidal grainstone preservedas calcite with some benthic foraminifer tests. (Lysite core, sequence IV, 110.33 fbmp [33.63 mbmp]).


cycles change abruptly, and evidence for subaerial ex-

posure exists (Figure 9). These sequence boundaries

are commonly marked by caliche crusts and microkarst

features, including small-scale solution-collapse brec-

cias. In outcrop, the tops of sequences I and II both

coincide approximately with the tops of cliff-forming

successions that are comprised by amalgamated grain-

stone beds. Above each of these two sequence bound-

aries, one high-frequency depositional cycle can be dis-

tinguished that is laterally very extensive and consists

of a complete transgressive-regressive cycle with trans-

gressive mudstones, including occasionally domal stro-

matolites capped by a thin regressive grainstone (Fig-

ures 5, 6). Our pick of the sequence boundary differs

from the pick of Crockett (1994), Moore et al. (1995),

and Sonnenfeld (1996a), who define the sequence

boundary in the mudstones overlying this high-frequency

cycle. We pick the sequence boundary below this cycle

because (1) there is evidence for subaerial exposure

at the base of this high-frequency cycle, but no evi-

dence for subaerial exposure on top of this cycle, and

(2) the high-frequency cycles below are composed of

Figure 8. (A) Dolomitized coarse skeletal grainstone composed almost exclusively of recrystallized red-algal debris (Buffalo Creek,sequence II). (B) Peloidal intraclast grainstone with large brachiopod shell (Lysite core, sequence IV, 120.01 fbmp [36.58 mbmp]).(C) Dolomitized caliche conglomerate at top of grainstone bed. Porosity is impregnated with blue resin, appearing gray (LysiteMountain, sequence II). (D) Evaporite solution collapse breccia, possibly reactivated by tectonic processes. Bitumen is present in openfracture. Porosity is impregnated with blue resin, appearing gray (Owl Creek, sequence II). (E) Karst-breccia accumulated on a cavefloor (‘‘parabreccia’’). Scale at left shows 1-cm (0.4-in.) increments (Lysite core, sequence IV, 125.00 fbmp [38.10 mbmp]).

Westphal et al. 417

amalgamated grainstones, whereas this cycle contains

a thick transgressive interval. Therefore, we interpret

the cycle as the first depositional event during the sea

level rise at the base of the next sequence. Consequent-

ly, we place the sequence boundary one and a half high-

frequency cycles deeper in the section compared to

Sonnenfeld (1996a, b). A gamma peak is recognized

above this first transgressive high-frequency cycle at

the Lysite A section in both sequences II and III (Fig-

ure 10). The gamma peaks occur approximately where

Sonnenfeld (1996a) picked the sequence boundaries.

Similarly, in the subsurface, the gamma peaks are used

to define the sequence boundaries (Crockett, 1994,

Moore, 2001). In our interpretation, the gamma peaks

are related to first flooding surfaces in sequences II

and III.

Figure 9. Regional correlation of the Madison Formation (sequences I– IV). The stacked grainstone units that were deposited intidally influenced high-energy shoals slightly increase in thickness toward the west of the study area. These stacked grainstones forma laterally continuous unit with few mudstone intercalations, resulting in a laterally homogeneous reservoir. Vertical exaggeration isabout 1:1000. The true depositional dip averages 0.034j.


Sequence I thickens westward in a downdip

direction, from about 50 to 85 ft (15 to 26 m), whereas

sequence II shows only a slight increase in thickness

from 63 to 69 ft (19 to 21 m). The two sequences display

a general upward facies evolution from high-frequency

cycles dominated by laminated mudstone and wacke-

stone with stromatolites into mainly wackestones with

solitary corals and, finally, into cycles dominated by

cross-bedded grainstones (see Figures 5, 6). The basal

mudstones-to-wackestones interval is placed in the trans-

gressive portion of the sequence. The high-energy grain-

stone packages are interpreted to be deposited during

the relative sea level highstand. Subaerial exposure at

both sequence boundaries is indicated by microkarst,

cementation reminiscent of caliche crusts, and/or red-

dish color.

The thickness of sequence III is approximately 113

ft (34 m) at the locations stratigraphically updip and

slightly increases to 127 ft (38 m) at the most downdip

measured section. Sequence III forms a recessive unit

with few good outcrops. The facies of this sequence is

very different to the underlying sequences. In the basal

part of sequence III, grainstones are present that in-

dicate high-energy conditions during early flooding, but

the bulk of the sequence consists of mudstones to wacke-

stones with chert nodules and crinoidal debris. Dedo-

lomitization and solution cavities are common. Sequence

III strata are commonly extensively brecciated. The most

dominant breccia type is calcite-cemented breccia that

is interpreted to be related to hydrothermal fracturing

during thrusting. Evaporite solution-collapse breccias

occur preferentially in the transgressive portions of

sequences III and IV. Karst breccias occur mostly in

sequence IV. The facies difference between sequences

II and III indicates a major seaward shift of the grain-

stone belt in sequence III. This downward shift can best

be explained by a large relative sea level fall on top of

sequence II, which is also the top of the first composite

sequence of Sonnenfeld (1996a). Indeed, sections fur-

ther down ramp (e.g., at Livingston and Baker Moun-

tain) contain grainstones at the base of sequence III

(Sonnenfeld, 1996a). Elrick and Read (1991) interpret-

ed these grainstone packages as ramp-margin wedges

that developed during long-term sea level fall and low-

stand conditions.

Sequence IV displays a 77-ft (23-m) thickness updip,

increasing downdip to some 160 ft (50 m). It is a gen-

erally regressive unit as indicated by thick grainstone

packages. The upper boundary is a major exposure ho-

rizon with large karst caves that cut as much as 300 ft

(100 m) deep into the formation. They are partly col-

lapsed or infilled with polymict karst breccias. The

depositional stratigraphy and facies are still preserved

at outcrops at Lysite. In contrast, the core at Lysite and

most of sequence IV at Buffalo Creek consist of polymict

karst infill breccias with a locally silty/sandy matrix.

High-Frequency Depositional Cycle (Genetic Unit)

Meter-scale, high-frequency cycles are the fundamen-

tal genetic units that build the reservoirs of the Mad-

ison Formation. These genetic units are the sedimen-

tary record of one cycle of creation of accommodation

space (Homewood et al. 1992). A transgressive and

regressive hemicycle can be recognized in each genetic

unit (Figure 11). The transgressive hemicycles are domi-

nated by tidal-flat (laminated mudstone and wackestone)

and subtidal deposits (e.g., stromatolites), whereas the

regressive portions are characterized by high-energy

carbonate sand shoal facies (Figure 11). In some cycles,

a horizon enriched with solitary corals separates the

two hemicycles. This coral-rich horizon is interpreted

to represent the turnaround from transgressive to the

regressive phase during a time of maximum water depth.

In the shallow-marine realm, a transgressive interval is

commonly recognized by fossil accumulations produced

locally as the transgression proceeded (Kidwell, 1985;

1989), and hardened ravinement surfaces commonly

act as substratum for benthic communities and corals

(Kidwell, 1983). The cycles are bounded on top by sharp

surfaces that, in most cases, are overlain by muddy strata,

indicating flooding (Figure 11). In the upper parts of

sequences I and II, the cycles consist of amalgamated

grainstone layers, and the cycle boundaries may be

marked by microkarst or reddish color (i.e., terra rosa)

that is indicative of exposure.

The different facies associations that develop dur-

ing transgressive and regressive phases are attributed to

facies partitioning (Homewood et al., 1992; Home-

wood, 1996). Facies partitioning is caused by different

energy levels, sediment supply, and preservation po-

tential, which develop during various stages of the cre-

ation and filling of accommodation space. During trans-

gression, the wave base progressively shifts shoreward

and creates a flooding or ravinement surface at the

upper slope and shoreline. Seaward of the ravinement

surface, where energy is low, fine-grained sediments

can be deposited and preserved. Preservation potential

of shoreline deposits is minimal because the approach-

ing sea constantly removes these deposits. During re-

gression, the preservation potential of sand bodies is

high as the energy level moves seaward. Consequently,

Westphal et al. 419

high-energy sand bodies are stacked laterally, and thick,

amalgamated reservoir units can be preserved.

In the Madison Formation, the facies are parti-

tioned between transgressive and regressive trends on

three scales: (1) the fundamental high-frequency cy-

cles, (2) the four third-order sequences, and (3) the

overall supersequence. Facies partitioning into trans-

gressive and regressive facies in each high-frequency

cycle varies according to the longer term trend.

Lateral and Vertical Continuity

On outcrop scale, the Madison Formation exhibits pa-

rallel bedding, with the thickness of third-order se-

quences, and even high-frequency cycles, being rel-

atively constant. The facies in each high-frequency cycle,

however, can vary laterally even on outcrop scale. The

lateral facies and subtle thickness variations are directly

related to the stacking of the genetic units in the third-

order sequences. High-frequency cycles in the early

transgressive portion of a third-order sequence are

highly continuous and can be correlated along entire

outcrops. They consist of a transgressive muddy or stro-

matolitic base and a high-energy grainstone top and

correspond closely to the architecture of a complete

cycle. Above the maximum flooding zone of the third-

order sequence, the lateral continuity decreases, and

mudstones to wackestones, stromatolitic buildups, and

grainstone bodies interfinger. Finally, in the regressive

part of the sequence, high-frequency cycles consist of

laterally continuous grainstone to rudstone bodies of

variable composition (e.g., red-algal rudstones to skel-

etal grainstones), and transgressive hemicycles are sparse.

The amalgamated grainstone layers in the upper part

of sequence II are especially continuous and laterally

extensive (Figures 5, 6). In contrast, the overall trans-

gressive nature of sequence III results in deposition

dominated by mudstone-rich transgressive hemicycles

with large lateral continuity (Figure 9).

On the regional scale, the 140-km (90-mi) transect

with the four outcrop sections displays a consistent fa-

cies architecture (Figure 9). At all locations, sequences

I and II are grainstone dominated, and sequence III is

mudstone dominated. The grainstone beds in the upper

parts of sequences I and II form a thick continuous sed-

iment body, although internally, the beds display

thickness variations (Figures 5, 6). The mudstones of

sequence II are laterally continuous over large distances.

Sequence III is dominated throughout the regional

transect by brecciated mudstones, the brecciation de-

creasing basinward. Sequence IV is dominated by lat-

erally extensive mudstones in the lower part and by

grainstones and floatstones in the upper part (Lysite

Mountain and Wind River Canyon).

The facies partitioning in the genetic units and the

sequences is also responsible for some of the vertical

heterogeneity in the formation. The mudstone intervals

in the transgressive parts of the cycles and sequences

generally have a lower porosity than the grainstone-

dominated regressive parts in outcrop and in the sub-

surface (Figure 10). Genetic units are commonly flow

units, but in the regressive intervals of the Madison For-

mation, grainstone-dominated genetic units amalgam-

ate to produce thick high-porosity intervals. As a result,

the entire regressive parts of the lower two third-order

sequences instead of individual genetic units are flow

units in the lower Madison Formation. The similarity

of the porosity profiles in outcrop and subsurface (Fig-

ure 10) indicates that the porosity partitioning in the

outcrop is also present in the subsurface. Additional

heterogeneity is introduced into the formation by dia-

genetic and tectonic events, which are discussed below.

DIAGENESIS

Figure 12 displays the paragenetic sequence of the

Madison Formation in the study area. Important for the

porosity distribution and, thus, reservoir quality in the

Madison Formation, are the distribution of dolomite

and the generation and destruction of porosity by dia-

genetic events. The following description focuses on

these points.

The earliest diagenetic alterations recorded are mi-

critic envelopes (Figure 12). Mechanical compaction is

manifest in some grainstones by fitted fabric, grain-to-

grain interpenetrations, and grain breakage, indicating

Figure 10. Correlation of sequences I to III and porosity of Madden well (BHP Petroleum 2–3 Bighorn), the Lysite shallow core, andone Wind River Canyon section. Sequence boundaries are shown as heavy lines. Gamma-ray curves show peaks around the maximumflooding zone and not at the sequence boundary. Porosity in the shallow core and the outcrop are from plugs, whereas the porosity logfrom the Madden well is from downhole measurements. Gamma-ray measurements were obtained with downhole tools. In the lowerpart, sequence boundaries are characterized by a decrease in porosity. Highest porosities in sequences I and II are located indolomitized grainstones. Rocks of sequence III show the highest porosities in the succession, but a less predictable porosity pattern.

Westphal et al. 421

a lack of an early marine cement framework. In most

samples, initial cementation took place synchronously

with mechanical compaction. Preserved early calcite

cements are restricted to sequence IV limestones. The

interpretation of these cements as compaction and

pressure-solution related is in agreement with Crockett

(1994).

Dolomitization is pervasive in the lower three se-

quences and shows little preferences for any precursor

facies with the exception of the most basinward locale

(Owl Creek), where some grainstones are preserved as

limestones, but all mudstones are dolomitized. Dolo-

mite is predominantly fabric destructive. Allochems in

many cases are discernible as faint ghosts or molds.

Dolomite is composed of euhedral (planar-e) to sub-

hedral (planar-s) dolomite rhombs forming a highly

porous sucrosic to tight dolomite. Dolomite crystal sizes

range from cryptocrystalline to more than 200 mm. The

average crystal size in dolomitized mudstones is about

45 mm, and in dolomitized grainstones, it is somewhat

larger (about 100 mm), indicating a relationship be-

tween crystal size and precursor material (Figure 13)

(Murray and Lucia, 1967).

On a regional scale, the Madison Formation in the

Wind River basin exhibits a characteristic dolomite dis-

tribution. The lower three sequences are thoroughly

dolomitized, whereas sequence IV is predominantly

preserved as limestones. In addition to this vertical

trend, dolomitization becomes less pervasive in a down-

dip direction toward the Owl Creek locale, but also

farther downramp, where an alternation between lime-

stones and dolomites is observed (Sonnenfeld, 1996b;

Elrick and Read, 1991; Smith et al. 2003). Dolomiti-

zation of grainstones, some of which are oolites, in all

updip locations of our study area contrasts with the

typical calcitic preservation of Mississippian oolitic grain-

stones on the North American mid-continent (Keith and

Zuppann, 1993). Moldic pores are particularly abun-

dant in the dolomitized grainstones in the upper parts

of sequences I and II. The dissolution is not related

to cycle or sequence boundaries. The formation of

the molds may have occurred during dolomitization or

Figure 12. Parageneticsequences in the MadisonFormation.

Figure 11. (A) General model of a high-frequency cycle (genetic unit) with its transgressive and regressive hemicycle (M =mudstone, W = wackestone, P = packstone, G = grainstone). (B) Example of a high-frequency depositional cycle at Buffalo Creek.Hammer for scale.

Westphal et al. 423

after dolomitization, when remaining calcite particles

were dissolved.

Solution brecciation in sequences III and IV fol-

lowed the dolomitization of sequence III. The clasts in

sequence III consist of finely crystalline dolomitized mud-

stones, some of which are lined by a second dolomite

generation that forms cement fringes. These second-

generation euhedral dolomite crystals exceed 500 mm

in size. In places, quartz cement occurs as linings around

dolomite clasts. It is followed paragenetically by calcite

spar, which has a similar petrographic appearance to

later generations of calcite. Solution brecciation is at-

tributed to the intense post-Madison meteoric influ-

ence because of exposure in the Pennsylvanian that

lasted for more than 20 m.y. and led to karstification

and erosion on top of the Madison Formation (Sando,

1967, 1988).

Deep burial as indicated by stylolites postdates the

formation of molds and precipitation of internal ce-

ments in molds that are cut by stylolites. Some stylolites

are oriented subvertically, which could reflect a Lara-

mide tectonic origin. Stylolites that crosscut calcite spar

indicate that at least one generation of calcite spar pre-

cipitated prior to deep burial.

The Laramide orogeny induced a second genera-

tion of fractures and brecciation. This brecciation over-

prints the entire Madison Formation but is strongest in

sequences III and IV. The tectonic breccia ranges from

fracture breccias with minor amounts of cement to

chaotic breccias with abundant blocky calcite cement.

In some breccias of sequence III, calcite spar occluding

the fractures between the clasts poikilotopically ex-

tends into the intercrystalline pore space of the sucrosic

dolomite of the clasts that also show dedolomitization

at their margins. In some samples, the partial dissolu-

tion of dolomite rhombs extends farther into the clast

than the calcite spar. Based on the morphology, dis-

tribution, and geochemical composition, these tec-

tonic breccias are hydrothermal in origin (Kislak et al.,

2001).

The tectonic/hydrothermal breccias in the Missis-

sippian carbonates of the Owl Creek thrust sheet, Wind

River Canyon, Wyoming, can be classified into four

categories based on fracture density, calcite volume,

and clast orientation (see above). The open fractures of

the tectonic breccia are healed by calcite precipitated

from the hydrothermal fluids (Figure 14). The calcite

cement precipitated has d18O values ranging between

�15 and �25xPDB, whereas the host rock values

range between �6 to +6xPDB. In contrast, the matrix

of karst-related breccias has relatively heavy d18O val-

ues (�5 to +2x). The light stable isotope values of

the tectonic breccia indicate a temperature of water

ranging from 80 to 120jC. The values that are constant

across the veins document no fluid evolution, suggest-

ing rapid precipitation (Figure 14).

We propose a multiepisodic fracture model for

the origin of these breccias. During thrusting, high-

temperature, high-pressure fluids from deeply buried

parts of the thrust sheet fractured the formation. The

open fractures were healed by calcite precipitated from

the hydrothermal fluids. This precipitation of cement is

most likely caused by rapid drop in pCO2. This process

was repeated in a series of small tectonically induced

Figure 13. Dolomite crystal size in all dolomitized samples and in dolomitized mudstones and grainstones to rudstones. Onlyunimodal dolomite samples were included. Note that dolomitized grainstone and dolomitized rudstone have slightly higher dolomitecrystal sizes than dolomitized mudstone. The dolomitized mudstone from the overall muddy sequence III has the smallest averagecrystal sizes.


events. The dense precipitated calcite heals most of the

fractures. As a result, fractures that would ordinarily in-

crease the permeability of a reservoir act to compart-

mentalize decreasing reservoir quality.

Bitumen in samples from the shallow core taken at

Lysite Mountain is found in open fractures, indicating

that hydrocarbon emplacement postdates tectonic

fracture-related calcite spar. Late fibrous calcite ce-

ment is found in molds and vugs in the outcrops. This

calcite is attributed to late meteoric cementation fol-

lowing the Tertiary uplift. As such, it has no bearing on

the reservoir quality of the Madison Formation buried

in the subsurface.

PETROPHYSICAL PROPERTIES

Porosity and Permeability

Porosity in the Madison Formation ranges from 0 to

35%. For the lower two sequences, the average porosity

is similar: 10.8% in the samples from the outcrop and

shallow core (LL&E 1A Madison Stratigraphic Federal)

from sequence I (n = 59), 12.1% in the outcrop and

shallow-core samples from sequence II (n = 77), and

12.1% in the deep-core samples (BHP Petroleum 2–3

Bighorn) from sequence II (n = 11). In sequence III

(outcrop and shallow core: 7.9%, n = 43; deep core:

Figure 14. Photomicrographs of a hydrothermal breccia and stable isotope values across calcite cement in the breccia. (A)Photomicrograph of fractured dolomite intruded by calcite cement. The clast-cement boundary is sharp, showing no dolomite growth.(B) Dolomite crystals growing on clast/cement boundary. (C) Transect shown in (D). (D) Stable isotope transect for a mosaic breccia.Consistent strongly negative values suggest rapid precipitation of cement at high temperatures.

Westphal et al. 425

7.3%, n = 7) and sequence IV (outcrop and shallow

core: 2.1%, n = 15), porosity values are distinctly lower.

Pore types include primary interparticle, moldic,

intercrystalline, and fracture porosity (Figure 15). Gen-

erally, the outcrop samples show a wider range of

porosity values than samples from the subsurface be-

cause the BHP Petroleum 2–3 Bighorn includes neither

the porous parts of sequence III nor the tight sequence

IV. Primary porosity is preserved exclusively in do-

lomitized grainstone and dolomitized rudstone, where

early dolomitization prevented considerable compac-

tion. This is especially true for the dolomitized red-algal

grainstone to rudstone units in sequence II at Buffalo

Creek, which have abundant preserved primary porosity.

Moldic porosity is the dominant type of porosity in

sequences I and II. It is present in about 53% of all

samples and in about 57% of the dolomitic samples.

Moldic porosity occurs in tight dolomites as well as in

porous, sucrosic dolomites. In tight dolomites, the molds

are isolated, whereas in porous, sucrosic dolomites, molds

are connected by intercrystalline pores. In such a case, a

good storage rock is combined with a permeable pore

system, considerably enhancing reservoir quality.

Intercrystalline porosity occurs in about 50% of

the dolostones of the Madison Formation. It is present

in all facies types but mostly in dolomitized mudstones

and is the dominant pore type in sequence III. Under

SEM, the individual rhombs and the well-connected

intercrystalline pores of sucrosic dolomites are clearly

distinguished (Figure 15). The degree of connectivity

decreases with the presence of tight, interlocking dolo-

mite crystal growth.

Permeability values range from 0.002 to 144 md,

with highest average permeabilities in rocks of se-

quence II (outcrop and shallow core: 21.92 md; deep

core: 43.20 md) (Figure 15). Sequence I samples ex-

hibit a slightly lower average permeability (outcrop:

13.63 md). The breccias of sequence III have low per-

meabilities (outcrop and shallow core: 3.24 md; deep

core: 7.24 md), whereas the fine sucrosic dolomites of

the sequence have a high permeability similar to the

dolomites of sequences I and II. The limestones from

sequence IV are characterized by very low permeabil-

ities (outcrop and shallow core: 0.03 md).

DISCUSSION

Reservoir-quality porosity in the Madison Formation

dolomites is dominated by the combination of moldic

pores that result in a high storage capacity and inter-

crystalline porosity that provides connectivity be-

tween the larger pores. The highest porosity is found

in sucrosic dolomite samples with intercrystalline po-

rosity (Figure 15). However, samples with moldic pores

and fractures can have porosities (and permeabilities)

similar to samples with intercrystalline porosity. The

presence of intercrystalline porosity in most of the

dolostones results in a rather homogeneous reservoir

with good porosity and permeability. We expect that

these facies have the highest production flow rates, al-

though this could not be confirmed from the operator.

Porosity in outcrop, shallow core, and deep core

shows similar trends in the stratigraphic column and

in a porosity vs. permeability diagram (Figures 10, 15).

Overall, porosity distribution shows a close relation-

ship to the sequence-stratigraphic architecture with a

diagenetic component. Dolomitized sequences I to III

have reservoir-quality porosity, whereas the calcitic

sequence IV lacks good porosity and acts as a seal. Var-

iations in the sequences are also related to the strat-

igraphic position. The regressive intervals form thick

intervals of porosity that are not interrupted by tight

layers. Highest porosity in sequences I and II occurs in

the packstones to grainstones of these regressive in-

tervals. The depositional facies of these packages is an

extensive high-energy shoal complex with grainstone

sandwaves intercalated with packstone intershoal fa-

cies. These facies heterogeneities are largely eliminated

by pervasive dolomitization that transformed these rocks

into porous, sucrosic dolomites with good intercrystal-

line connectivity. The sequence boundaries, in contrast,

are in most cases less porous, which is a result of tighter

caliche horizons and calcified small-scale breccias. In

addition, the basal, transgressive mudstones to wacke-

stones of each sequence are less porous.

The highest porosities in the Madison Formation

occur in sequence III. These high-porosity intervals,

however, cannot be correlated between well locations

in the subsurface. Outcrop observations indicate that

these lateral heterogeneities can be attributed to the

extensive brecciation of sequence III (see below).

Fracturing occurs on several scales in the Madison

Formation and also changes upsection. In sequences I

and II, large vertical fractures have a spacing of about

15–65 ft (520 m). They are crosscut by smaller inclined

fractures. Fracturing can be expected to contribute sig-

nificantly to the vertical flow in these two reservoir

units. In several Madison fields in the area, such as the

Elk basin and Garland, fracture porosity of this type is

responsible for wells with very high flow rates (Lorenz

et al., 1997).


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Westphal et al. 427

In sequence III, the fracture style is dominated by

shattering, with random fractures creating small cen-

timeter- to decimeter-scale blocks. The light stable

isotope values document that this type of fracturing

is related to hydrothermal brecciation, producing a

monomict dolomite breccia with calcite cements sur-

rounding the clasts. The calcite cements precipitated

shortly after brecciation from the hot fluids and isolate

individual clasts from each other. These healed frac-

tures reduce porosity in the zones of brecciation that

can be tens of meters wide and compartmentalize the

otherwise high-reservoir-quality dolomite of sequence

III. The hydrothermal brecciation is also observed in

sequences I and II, but to a lesser extent. The reason

might be that the hot fluids that explosively entered the

strata preferentially used sequence III.

Factors Controlling Permeability and Porosity

There is a general trend (correlation coefficient: r2 =

0.409) of higher permeability with increased porosity,

although some variability exists (Figure 15). The cor-

relation reflects the fact that the reservoir is composed

of sucrosic dolomite with interconnected intercrystal-

line pores and not of limestone, where isolated, in-

effective pores commonly obliterate a good correla-

tion. Nevertheless, there is a considerable range of

permeability for a given porosity. For example, high

permeability of more than 20 md is found for porosity

ranging from 8 to 26%. Likewise, samples with po-

rosity of approximately 10% show a range of perme-

ability from 0.05 to 50 md. Several factors influence

the porosity-permeability relationships, resulting in

these deviations. They include the following:

1. Precursor sediment: Pore structure is partially re-

lated to the precursor sediment. Both dolomitized

mudstones and dolomitized grainstones exhibit a

range of pore structures and, thus, permeabilities.

However, a plot of the frequency of dolomitized

grainstones and dolomitized mudstones vs. porosity

shows a subtle trend to higher porosity for the

dolomitized grainstones (Figure 16).

2. Mineralogy: The highest values of both porosity

and permeability are observed in dolomitized grain-

stones of sequences I and II and in sucrosic dolo-

mites of sequence III, whereas the limestones from

sequence IV have very low porosity and permeabil-

ity. Taking 5% porosity and 0.01 md permeability as

a cutoff for reservoir quality, 95% of the samples

from the calcitic sequence IV are in the nonreservoir

range, whereas 47% of the dolomitic, lower three

sequences plot in the reservoir-quality field.

3. Pore structure: Porous, sucrosic dolomites display

the highest porosity and permeability and a good

porosity-permeability relationship as a result of con-

nected pore structure. Tight dolomites and lime-

stones contain isolated pores that do not contribute

to permeability and, thus, effective porosity.

4. Dolomite crystal size: In the Madison Formation,

porosity and permeability do not directly depend on

the crystal size of dolomite (Figure 17) as would be

expected for pure dolomites (Lucia, 1995). The

absence of a correlation is the result of (1) abundant

moldic porosity in the Madison samples, (2) dif-

ferent degrees of interlocking of the dolomite crys-

tals (i.e., intercrystalline-porous vs. tight dolomite),

and (3) most importantly, irregularly distributed

postdolomitization cements that destroy a possible

former relationship between crystal size and porosity-

permeability.

5. Calcite cement: Most of the variability in the po-

rosity vs. permeability plot is caused by the local

closure of pore throats by late calcite cements that

are present in the subsurface as well as at the surface.

The amount of calcite cement is highly variable and

unevenly distributed in the strata. High-porosity rocks

generally lack the late calcite cement. In samples with

Figure 16. Porosity in dolo-mitized mudstones and grain-stones to rudstones. Dolomitizedgrainstones and dolomitizedrudstones show slightly higherporosities than dolomitizedmudstones.


high permeabilities and intermediate porosities, some

pores are occluded by calcite, whereas others lack

calcite cement. The amount of calcite cement does not

affect porosity and permeability equally. Whereas

calcite cement decreases the porosity by occluding

the intercrystalline and moldic/vuggy pore space,

permeabilities appear to be less effectively reduced.

Obviously, a certain threshold needs to be reached

before the calcite cement inhibits connectivity. This

threshold is dependent on the precursor pore

structure. Large intercrystalline pores require more

calcite cement for complete occlusion than small

interparticle pores. Most of the calcite cements are

of late diagenetic origin and occupy pore space in

the previously pure dolomites. We suspect that this

late calcite is precipitated from hydrothermal wa-

ters that healed the fractures in the hydrothermal

breccia zones as these waters partly invaded the

formation.

6. Fractures: Several samples plot in the low-porosity/

high-permeability area of the porosity-permeability

crossplot, suggesting that the samples are fractured.

Fractures are common in the Madison Formation,

and microfractures were observed using the SEM.

Flow Barriers and Compartments

Three large flow units are recognized in the Madison

Formation (in both outcrop and the subsurface; C.

Hawkins, 1998, personal communication). They ap-

proximately coincide with the lower three third-order

depositional sequences. The fact that samples close to

sequence boundaries appear to act as seals (C. Haw-

kins, 1998, personal communication) implies that the

flow units are third-order sequences instead of the high-

frequency cycles (Figure 18).

In sequence II, the grainstone-dominated parts

show the most consistent high porosity. However, po-

rosity is not equally distributed in the sequence. Po-

rosity is highest in the middle to upper part of the

sequence, where dolomitized grainstones are present,

but decreases to very low values toward the top. Po-

rosity is clearly related to the grainstone facies belt,

where moldic porosity is combined with intercrystal-

line porosity. Porosity reductions are caused by dia-

genetic effects related to the sequence boundary such as

chertification and caliche that result in a flow barrier

toward the sequence boundary. Additionally, differ-

ences in facies in the sequence itself cause porosity var-

iations. The basal high-frequency cycles of the sequence

are dominated by mudstone and stromatolitic layers

that have a lower porosity.

The high lateral continuity, extensive distribution,

and favorable petrophysical properties of the high-energy

grainstone bodies of the regressive parts of sequences I

and II make them a high-quality reservoir. The high-

quality grainstone reservoir facies appears to occur in

the same stratigraphic position over a distance on the

order of 100 km (60 mi) in a downslope direction (east-

west), potentially forming a continuous layer; however,

the pervasive dolomitization is ultimately replaced down-

dip by more facies-sensitive dolomitization that leaves

the grainstone bodies largely undolomitized (Smith et al.,

2003). Sequence III consists of the most homogenous

facies, yet the porosity distribution is the least predict-

able because of extensive hydrothermal brecciation

and the sealing of clasts by calcite cements. Judging

from the outcrops at Lysite Mountain, the brecciation

zones might be as wide as 100 m (300 ft), with a re-

currence of approximately every 100 m (300 ft). How-

ever, the distribution varies widely according to the posi-

tion in the individual thrust sheets. Preliminary data

indicate that the frontal portions of the thrusts are more

likely to be affected than the rest of the thrust sheet.

The limestones of sequence IV exhibit overall low

porosities, and hydrothermal brecciation appears to

have less effect on this sequence, but locally, karst brec-

cias make up most of the sequence. On a large scale,

sequence IV is the flow barrier on top of the Madison

Formation.

Sequence boundaries with their exposure-related

features seem to act as flow barriers over intermediate

distances, but probably do not isolate the flow units

Figure 17. Porosity-permeability plot of samples that consistof unimodal dolomite. The crystal size groups are marked bysymbols. For discussion, see text.

Westphal et al. 429

entirely. Internally, the sucrosic nature of the dolomite

is thought to result in laterally and vertically extensive

flow units. The amalgamated high-energy grainstones

of the regressive parts of sequences I and II probably

act as large, continuous reservoir units. Local reduction

of porosity and permeability by late calcite cement

would probably have minimal impact on the overall

reservoir performance. In contrast, the calcite lining of

the dolomite clasts in sequence III breccias has a major

impact on reservoir heterogeneity.

Figure 18. (A) Elements creating reservoir heterogeneity in the Madison Formation. High-frequency depositional cycles with low-energy mudstone to wackestone at the base and high-energy grainstone on top produce initial heterogeneity. Pervasive dolomitizationin sequences I– III obliterates most of this heterogeneity, but brecciation caused by solution collapse and later karst introduces newheterogeneity. Hydrothermal brecciation and associated calcite cementation of clasts and pore space reduces reservoir quality duringthe Laramide thrusting. (B) Schematic distribution of reservoir heterogeneity in the Mississippian Madison Formation. Sequences I– IIIconsist of reservoir-quality strata, whereas the limestones of sequence IV act as a seal in the subsurface. Within the first three sequences,well-cemented beds around the sequence boundaries have low permeability, providing vertical heterogeneity. Late-stage calcite cementthat surrounds the clasts of the hydrothermal breccias and invades the porous dolomite creates horizontal flow barriers, especially insequence III.


CONCLUSIONS

The sedimentary record of the Madison Formation

clearly shows that cyclic deposition resulted in stacking

of high-frequency depositional cycles (genetic units)

in depositional sequences. However, this small-scale

stratigraphic architecture has only limited influence on

the petrophysical properties of the formation.

The depositional environments of the three basal

sequences display a similar pattern. On a large scale, the

basal portions of the two lowermost sequences com-

prise mudstone, algal mats, and, locally, stromatolites.

Mudstone units are laterally continuous over long dis-

tances, whereas the stromatolites show a more variable

distribution. These facies are overlain by stacked grain-

stone units that were deposited on tidally influenced,

high-energy shoals. Individual grainstone bodies are

amalgamated to form a thick, continuous sediment body

of constant thickness over a distance of approximately

100 km (60 mi). On outcrop scale (i.e., reservoir scale),

these stacked grainstones are laterally continuous grain-

stone bodies with few mudstone intercalations. This

lateral continuity and lack of vertical separation of the

facies belt, in conjunction with pervasive dolomitiza-

tion, produces extensive carbonate units with good res-

ervoir quality throughout the study area, i.e., more than

approximately 100 km (60 mi).

Variations of the reservoir quality in the Madison

Formation are the result of a combination of (1) cyclic

stacking of reservoir facies, (2) a diagenetic overprint, in

particular, dolomitization that largely follows the dep-

ositional units, and (3) a late-stage hydrothermal brec-

ciation and calcite precipitation. The stacked grainstone

cycles of the regressive parts of sequences I and II are

the main flow units. Variations in the high-frequency

cycles are of little importance as flow barriers. The tight

lithologies at the sequence boundaries and in the bot-

tom of the sequences act as aquitards. Thus, the flow

units (compartments) are developed at a sequence scale

with a thickness of approximately 50–100 ft (15–30 m),

not at a high-frequency cycle scale. The diagenetic over-

print, especially dolomitization, is important for pro-

ducing reservoir-quality strata, because most of the

dolomitization in the Madison Formation is porosity

enhancing, resulting in high initial porosities and per-

meabilities in grainstone packages of the regressive parts

of sequences I and II and the mudstone interval of

sequence III. These high-reservoir-quality rocks consist

of well-developed intercrystalline porosity with high

permeability. Dolomitization is pervasive in the lower

three transgressive sequences. The fourth regressive

sequence is largely preserved as limestone. The absence

of dolomitization in the fourth sequence of the Mad-

ison Formation results in an effective seal on top of the

reservoir.

Later diagenetic processes introduce heterogene-

ities into the reservoir-quality dolomites of the lower

three sequences. Most important for reservoir hetero-

geneity is the hydrothermal brecciation that occurred

during the movement of the thrust sheets. Although

the brecciation fractured the rocks extensively, subse-

quent precipitation of calcite cements healed the frac-

tures and isolated highly permeable clasts and strata

from each other in a random pattern. Furthermore, hy-

drothermal fluids locally invade the permeable forma-

tion and occlude porosity and permeability in a way

that is hard to predict.

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Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

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